1. Field of the Invention
The present invention relates to a zoom lens system and an electronic imaging apparatus using the same.
2. Description of Related Art
In the digital camera market, while there is a tendency for further miniaturization and a higher zoom ratio, there also is an increased demand for a higher definition (higher picture quality). One way to obtain a higher definition is to, for example, use an image sensor (image pickup device) that has a large APS size. In an SLR camera, the image sensor is large, so that a high definition image can be achieved, however, since the overall camera system is large and heavy, demands for further miniaturization cannot be met. In recent years, a so-called “mirrorless” SLR camera has been proposed in which the quick-return mirror, which is a major characteristic feature of SLR cameras, is removed and does not use an optical finder system; however, it cannot be said that such mirrorless SLR cameras achieve sufficient miniaturization. Furthermore, lens-shutter zoom lens systems which pursue miniaturization have also been proposed, however, the incidence-angle of light that is incident on the imaging surface is not sufficiently perpendicular to the imaging surface (i.e., has poor telecentricity), so that use of such lens-shutter zoom lens systems with a digital imaging sensor is not realistic.
Conventionally, a zoom lens system is known in the art which has three lens groups, i.e., a negative first lens group, a positive second lens group, and a negative third lens group, in that order from the object side, for use with an image sensor having a large APS size. In this conventional zoom lens system, it is typical for the third lens group to be configured of two lens elements, i.e., a negative lens element and a positive lens element, in that order from the object side, or a positive lens element and a negative lens element, in that order from the object side. Examples of the latter arrangement of the third lens group can be found in Japanese Unexamined Patent Publication Nos. 2001-290076, 2002-221660 and 2009-25534.
However, although the former zoom lens system in which the third lens group is configured of a negative lens element and a positive lens element, in that order from the object side, is advantageous for improving telecentricity, various aberrations such as lateral chromatic aberration, astigmatism, distortion and spherical aberration that occur in the negative first lens group cannot be favorable corrected. Furthermore, since the negative lens element on the object side within the third lens group and the positive lens element on the image side within the third lens group constitute a local retrofocus lens arrangement, an increase in the overall length of the zoom lens system cannot be avoided.
Whereas, in the latter zoom lens system in which the third lens group is configured of a positive lens element and a negative lens element, in that order from the object side, in order to secure the negative refractive power of the third lens group, the refractive power of the negative lens element on the image side needs to be increased, so that the refractive-power balance between the positive lens element on the object side and the negative lens element on the image side is not favorable.